If I didn’t know any better, I’d say Mickey and Billy borrowed this segment from Piano Piece and incorporated it into part of their “drum” segment. Sounds like it could be on Anthem of the Sun. I know I’ve heard this at many a show. And the piano is considered a percussion instrument after all.

Not exactly easy listening, but worth it for the historical value. It's like the birth of "space". A very good sounding recording, especially considering the lineage. The amount of work you put into this shows, sir. Never knew this existed, but a search revealed this info from http://deadessays.blogspot.com/2010/07/constanten-on-constanten.html :
I got back to the States in 1964 I moved to the Bay Area."
"Phil and I had our first LSD from the same batch in early '64, when it still came on sugar cubes."
"I played with Phil, most notably at a 1964 concert, under the auspices of the San Francisco Mime Troupe, that featured Phil, Steve Reich, and me. Phil contributed a quasi-concerto for prepared piano called 6 7/8 for Bernardo Moreno, with me on piano. We gave four performances, each unique (although the third night was especially magical, in my memory)."